1. Field of the Invention
The invention relates to a process for controlling the metering of metal in die-casting machines in which the metering is effected by pressure reduction.
2. Discussion of the Related Art
EP 0,051,310 B1 has disclosed a die-casting machine which operates using the so-called vacuum die-casting process. In this process, the molten metal is sucked out of a holding furnace, via an intake pipe, into the casting chamber by means of pressure reduction, the pressure reduction being applied via an extraction duct in the mold-parting plane of the casting die. The pressure reduction applied to the casting die is to fulfill substantially 2 functions: Firstly that of degassing the casting chamber and the die and secondly that of sucking the amount of metal which is required to produce the parts out of the holding furnace into the casting chamber. The metering accuracy represents an essential factor for the quality of the castings. A high metering accuracy also requires suitable control of the machine parameters in order to achieve the desired process reliability.
A proven process for good metering accuracy is disclosed by DE 41 12 753 A1. By means of a measuring device, the filling level in the casting chamber is measured by a sensor. However, this process is only suitable for cold-chamber die-casting machines, in which the metering is effected, for example, by a metering ladle into an opening of the casting chamber. The function and the measurement accuracy of a sensor for level measurement is described extensively in EP 0,014,301. A die-casting machine which is operated using the vacuum process operates in the manner of a closed system, i.e. there is no external metering. A metering aperture in the casting chamber, as described in DE 41 12 753, which is generally directed upward, is not present, and the procedure of measuring the filling level in the casting chamber consequently cannot be employed without problems.
The essence of the invention consists in further developing the process from DE 41 12 753 A1, in the name of the inventor, in such a way that it is possible to use this .process in vacuum die-casting machines. Since a level measurement in the casting chamber cannot be carried out easily, the corresponding measurement is carried out in the holding furnace. For this purpose, a probe is fitted in the holding furnace to determine the filling level of the molten material and the change in this level during the metering phase. Since the invention is not restricted to measuring a level change, but rather proposes an entire control circuit, an actual value is formed from the signal from the probe and this value is compared to a desired value. The desired value is determined from the parameters required for optimum production of the parts and is provided with permissible tolerances. The result of the comparison of the desired value and of the actual value is processed in a computer in such a manner that metering parameters, such as for example pressure reduction and metering time, can be set for optimum production of castings. The computer contains mathematical and physical formulae and rules relating to this control process, and these formulae and rules are supplemented by specialist knowledge from the casting sector. In this way, the computer is able to determine the optimum process parameters at any given time and to transmit the values to the machine control unit in order to carry out control operations. The level measurement may be supplemented by further measurement parameters.
By way of example, the filling level of the furnace can be determined using the furnace weight, or the temperature-dependent viscosity of the molten metal can be determined by suitably evaluating a temperature measurement. Monitoring of the suction time is also provided for at a vacuum valve. If a desired value is exceeded, this is an indication of an operating fault or of incorrect production of parts, if the required metering quantity in this period has not been confirmed by a level sensor. All these measures serve to increase quality and therefore to minimize reject parts. Since the entire casting process is characterized by a large number of influences, it is important to control the individual parameters reliably. For example, not only are the geometry and microstructure quality of the casting dependent on the metering accuracy, but, to achieve them, some setting parameters of the die-casting machine are too. By way of example, this applies to the changeover points of the pressure- or displacement-dependent connection of the individual casting phases, and knowledge of the temperature and viscosity of the molten metal is also required to control the casting rate and the specific casting pressures. Introducing specialist knowledge from the die-casting sector in combination with the use of a computer also allows significantly more complex analysis of the actual data and their suitability to be carried out. For example, a molten material temperature which is supposedly too low can still lead to good parts by increasing the pressure reduction and therefore reducing the metering time. Specialist knowledge from the die-casting sector also includes knowledge of the fluid dynamics of the molten metal. Therefore, in the suction and metering phase a high vacuum of, for example, 50 mbar is desired, with the result that favorable inflow rates of approx. 4÷10 m/s occur in the region of a restrictor which is arranged in the inflow region of the intake pipe.
A high level of process reliability can be achieved with little outlay using the proposed metering process. Advantageous developments and improvements of the process according to the invention are given in the subclaims.
Further details and advantages are explained in more detail in the following description of an exemplary embodiment.